Tests that measure the patient's ability to control his posture generally fail to yield much clinically useful information about vestibular function. The reason for this failure was made explicit by an analysis of human postural control performed by Nashner. He showed that postural control does not depend on vestibular cues when visual and kinesthetic cues are available. To test vestibular function, one must force the patient to rely on vestibular cues by eliminating cues from these other sensory systems. We are developing a method for evaluating vestibular control of posture based on Nashner's analysis. Vision is eliminated by eye closure and kinesthetic cues are eliminated by rotating the supporting surface to null changes in ankle angle. Body sway is monitored by a video/computer system and torques generated by postural adjustments are monitored by semiconductor strain gages mounted on steel flexures that support a platform upon which the patient stands. We propose to develop and validate a vestibular function test, based on this method, that will make discriminations between vestibular and non-vestibular lesions, between semicircular canal and otolithic lesions, and between right-sided and left-sided lesions.